Hybridized polymers as bioactive bone substitutes and GTR scaffolds

Il testo sottostante descrive l'attività di ricerca e studio di biomateriali e di processi altamente innovativi per creare un supporto di impianto a lungo termine e ottimizzarne le caratteristiche allo scopo di fornire un materiale altamente personalizzabile e che sia vincente nella rigenerazione tridimensionale dei tessuti come una struttura attiva, biodegradabile, eterogenea e anisotropica.


State of the art

Bone defects occur in a wide variety of clinical situations, and their reconstruction to provide mechanical integrity to the skeleton is a necessary step in the patient's rehabilitation. The current gold standard for bone reconstruction, the autogenous bone graft, works well in many circumstances. However, autograft reconstruction, along with the available alternatives, do not solve all instances of bone deficiency. Recently, novel materials, cellular transplantation and bioactive molecules delivery are being explored alone and in various combinations to address the problem of bone deficiency.
Development of hybrid polymer systems (copolymers, complexes, hydrogels, blends, etc.) based on natural and synthetic macromolecules and their open wide spectrum of applications in the biomaterials science has received tremendous attention. A variety of implantable devices have been studied for the sustained release of biologically active compounds, and recently tissue engineering has shown great promise for creating biological alternatives for implants using hybridized polymers.
Calcium phosphates are generally considered materials of choice as bone substitutes. While calcium phosphate ceramics meet some of the needs for bone replacement they are limited by their inherent stiffness, brittleness and low fatigue properties relative to bone and are generally not resorbed during bone remodelling. Despite these problems, macroporous calcium phosphates have been adopted as scaffolds for the restoration of bone through bone tissue engineering strategies. So low-density hydroxyapatite with highly interconnected porosity has also been advocated as a viable alternative to bone grafts without the complications of strerilisation, infection, rejection and inadequate supply. Furthermore, porous structures invite ingrows of bone into implant, leading to a more securely fixed and integrated repair, particularly in cancellous bone where the structure closely mirrors that of the host.
Some bioresorbable polymer mixtures with hydroxyapatite have been proposed for use in bone repair, because they include bioactive osteogenesis. The production of a very biocompatible composite material, constituted with a biointegrable polymeric matrix and a ceramic, has been considered to be useful for applications in which an initial shape is easily obtained through typical plastics processing. The moulding technology should allow reproduction of a part of bone with standard devices at low or higher temperatures, cheaply and with no host's need of close tolerances, because the final remodelling is left to the tissue itself, which guests the implantation. Bone reconstruction should follow resorption of the polymer and should nucleate all around each grain of ceramics, in particular if the latter is bioactive.
Osteoconduction appears to be optimized in biomaterials that mimic not only bone structure, but also bone chemistry. The osteoconductive matrices in which hydroxyapatite grains are anchored will be synthesized by crosslinking collagen conjugated with hyaluronan i.e., poly-L-lactide (PLLA). The resulting three-dimensional scaffold which adheres to both hard and soft tissues has sufficient wet cohesion in body liquids. This patented hybrid type HA-COL-HYA (or PLLA) composite with designed macrostructure provides programmable and more complete bone proliferation and vascularization. In order to reach this target, the crosslinking agent and optimum composition have been selected. The excellent biological properties of this hybrid composite were confirmed in preclinical testing.
Composites consisting of biodegradable polymers (including starch, poly-L-lactide; polyhydroxyalcanoates; hyaluronan and collagen) in combination with hydroxyapatite (HA), three-Ca phosphate (TCP) or bioactive glass reinforcements, will be processed in order to generate both bone and periost substitutes with enhanced biological and mechanical (functional) properties. Biosynthetic membranes and skin substitutes based on modified collagen-chitosan-hyaluronan complex will be created for use in wound healing and guided tissue regeneration (GTR). Hybrids with 3D cell osteogenic cultures of bone marrow and neurogenic NGF bioactive factor and other neurotrophins, will be developed in order to generate heterogenous multy-functional implant biomaterials.
The interplay between hard tissue and nerve tissues has been documented at the molecular level by (Lerner,1992), who found neuropeptide containing fibres in bone and functional receptors of bone cells. A neuro-osteogenic “interaction” was accordingly proposed by Lerner (1992), who subsequently stated, in a review on kinins and neuropeptides, that neuropeptides directly or indirectly may modulate the activity of bone cells in physiological and pathological conditions (Lerner,1996). Neuro-osteological evaluation in medicine and dentistry, of pre-natal and post-natal pathological developmental conditions, provide obvious possibilities for neuro-osteological combinations of CNS and osseous findings. A possible osseous malformation such as a cranio-paryngeal canal, diagnosed from CT scans, may thus indicate the site of abnormal brain/cranial base development (Kjaer and Russell,1995; Kjaer, 1998) ect. Nerve growth factor (NGF) is an endogenously-produced neurotrophic factor which plays a crucial role in growth, differentiation, survival and function of neurons in the systems peripheral and central nervous system (CNS). Recent studies indicate that NGF promotes recovery of several neurological deficits and stimulate wound healing in cutaneous tissues. These NGF effects are mediated by two well-characterized transmebranes glycoproteins, the high-affinity (trkA) receptor tyrosine kinase and the low-affinity (p75) receptor. There is also consistent emerging evidences indicating that NGF either alone or synergistially with other biological endogenous mediators play a crucial role in cartilagineous and bone tissues. This hypothesis is suggested by recent studies showing that exogenous administration of NGF improves fracture healing in laboratory animals, stimulates osteogenesis and increases the rate and quality of fracture repair. The observations that chondrocytes express NGF and TrkA receptors and that NGF is elevated in synovial fluid of patients with chronic arthritis, juvenile chronic arthritis, in cartilage-related diseases and that this hypothesis is consistent with the observation that the basal NGF levels in osteoarthritis is associated with the distribution of neuropeptides, such as substance P (sP) and Calcitonin gene-related peptide (CGRP), which are implicated in this pathology.
Based on these findings it is highly possible that NGF exerts an important therapeutic implications for bone regeneration and for use in skeletal reconstruction and joint replacement.

Objectives:

Specification a new methodologies, functional tests, optimisation of production, modelisation and reproduction of complex synthetic composite biomaterial, resorbable and biocompatibile for achievement of more complete bone and nerve ingrowth in orthopaedics and dentistry.Objectives The originality and innovative character is based on:

1. Development the hard tissue substitute based on hybrid polymeric systems (PLLA, chitosan, hyaluronan, collagen) mentioned in state of the art, with hydroxyapatite, which mimic macrostructure and chemical composition of bone. Such designed bone substitute provides programmable and more complete bone proliferation and vascularization. A special processing technologies will contibute to production of composite and enhancement of it’s mechanical properties as a bone substitutes. Metodologies such as: twin-screw extrusion, injection moulding, Overmoulding and Sandwich moulding, SCORIM, biomimetic coatings, fabrication of porous architectures, stiff composites and/or nano-particles will be used for process optimization, modellisation and replica applications on replancement/fixation of bone.
2. This new multy-functional composite should be successful in three-dimensional tissue regeneration as being biologically active, biodegradable, heterogenous, anisotropic structures. The composite has been hypothesized to fulfil ideal charachteristics of a bioresorbable material, to restrict invasion of fibrous tissue to a bone defect, but to facilitate nerve ingrowth (innervation) together with osseoinduction.
3. The material must be capable of reproducible synthesis, should retain sufficient strength, should be no sustained to inflamatory reactions or foreign body response, and should be completely resorb with no histological evidence of residuals.
4. NGF would effects and promote both nerve ingrowth and bioactive bone tissues during the initial time of wound healing. This study will be carry out in vitro using tissues culture methodologies for understanding NGF action on specific bone cell population (osteoblasts) and to identify mechanisms involved in the cascade of events that can lead to both tooth development and/or tissue repair. Animal models of tooth diseases and bone fractures will be used to assess the action of exogenous administration of NGF and/or other neurotrophins in tooth development and repair processess. Our recent report showing that NGF is able to promote healing in human corneal and pressure ulcer and in human vasculite suggests that the utilization of NGF might be useful to set up a new tissue-engineered bone repair.
5. Priorities during production of new materials will be cost and efficiency improvements and better and safer procedures. Such objectives will contribute to the EC social objectives by – improving the quality of life both the reducing neccessity of implants replancement and reducing the ossification and healing time. Sustaining imployment - new technology and product(s) will have high potentials in future and will develop European competitiveness in field of production of dental and orthopaedic prosthesis.